A procedure for applying one or more printing inks to material, for example material in the form of a web, includes a step of drying the printing ink or inks by means of UV-radiation. In one fore of UV-radiating apparatus, as is disclosed in DE-A-39 02 643, for irradiating printing inks on material in web fore which is continuously transported past the apparatus, it is possible for a reflector housing which is of a substantially two-part configuration to be pivoted in its entirety about an axis which is disposed in its interior and which extends parallel to the longitudinal axis of the radiation source, from an operative position into a waiting position in which the reflector housing shields the web of material from the radiation which is emitted from the radiation source. The aim of making the reflector housing rotatable in that way is to render redundant the usual flaps or shutter members which are generally disposed on the UV-radiating apparatus at the underside thereof and which are moved into a closed position, to prevent the web of material from being overheated by the heat radiation effect, whenever transportation of the web of material has to be interrupted. Although the above-described pivotable arrangement of the reflector housing makes it unnecessary to provide shielding flaps or shutters, it does however suffer from the disadvantage of requiring an extremely large amount of space as the reflector housing is pivoted in its entirety through an angle of 180.degree.. In addition, the execution of such a pivotal movement on the part of the reflector housing takes up a certain amount of time so that such movement can only be produced in exceptional operational situations, such as for example when the web of material has come to a halt as mentioned above.
An UV-radiating apparatus which is of a somewhat different configuration and which is also employed for drying UV-responsive inks on material in web form is disclosed in EP-A-0 222 060. In this case also, to reduce the effect of the heat radiation on the web of material, the apparatus has a cold-light mirror as a reflector which reflects UV-rays but which allows a large part of heat rays to pass, which are then absorbed by some parts of the housing. The reflector housing of that apparatus is also of a substantially two-part configuration, the two halves of the reflector busing being adapted to be pivotable relative to each other. However that pivotability only serves to provide for focussing of the reflected radiation. Shielding of the material in web form, when the material is stopped, is effected by means of a shutter member which is closed when the web of material is stationary and thus shields the web of material from the radiation source.
An UV-radiating apparatus for drying printing ink is also disclosed in DE-A-22 35 047, including a reflector housing having side portions which are each pivotable about an axis parallel to the longitudinal axis of the radiation source, in such a way that in the one limit position they close the reflector housing at the underside thereof and thus shield the web of material relative to the radiation source when the web of material is stationary or when the speed of transportation movement thereof is too low. In that arrangement the middle part of the reflector housing is stationary so that it always remains in its stationary position. So that the heat which is generated when the reflector housing is in a closed condition can be suitably removed therefrom, cooling air is blown through the reflector housing. In addition, when the reflector housing is in the closed condition the radiation source is operated at a reduced power level in order to prevent overheating of the reflector housing which is still closed at its top side.
It will be seen that the above-discussed procedures, the content of which is appropriately incorporated hereinto by reference thereto, are concerned with printing on continuously transported material in web form. The web material which is provided with printing ink is shielded only whenever the web comes to a halt or is transported at an excessively slow speed. In all cases what is involved is preventing an excessively strong action on the part of the heat rays. As in normal operation the web passes the UV-radiating apparatus at an at least substantially constant speed, it is possible for the period for which the UV-rays act, and thus the metering thereof, as well as the period for which the long-wave heat rays act, to be influenced within certain limits by way of the speed of transportation movement of the web. The speed of movement of the web can be so selected that on the one hand overheating of the web as it moves in the region of the UV-radiating apparatus is avoided, while on the other hand the period for which the UV-radiation acts is adequate to dry or polymerize the printing ink.
However the need for drying printing ink by means of UV-radiation also arises in procedures involving printing on material which is transported with a stepwise movement. Such material may also involve material in web form which is advanced with a stepping movement, or individual articles such as bottles, CDs, and other hollow bodies. Under normal circumstances, in that situation also the arrangement is such that the UV-radiating apparatus or apparatuses is or are integrated into the actual printing machine, for example in such a way that an UV-radiating apparatus is arranged downstream of at least one printing station, as considered in the direction of transportation movement.
When dealing with individual articles, the procedure involved is generally such that a transportation means, for example a chain conveyor, which operates with a stepwise motion, is disposed beneath the UV-radiating apparatus. The transportation means moves the respective article or the respective web portion to be irradiated into a position beneath the UV-radiating apparatus, leaves it in that position for a certain period of time and, after termination of the appropriate treatment, moves it away with the next step in the transportation motion. At the same time the following article or web portion is moved into position beneath the radiating apparatus for irradiation thereby. In that respect the period of time for which the article is disposed beneath the UV-radiating apparatus can be fixed by the time which is required to achieve the desired effect, that is to say hardening or setting of the printing ink or inks, with a given level of radiation intensity. It will be noted however that, when a drying station which includes an UV-radiating apparatus is part of a larger piece of equipment, besides other treatment stations, for example in such a way that, in a printing machine, a drying station is disposed downstream of each printing station and thus the transportation means not only transports the articles through a drying station but also passes the articles in succession through a plurality of treatment stations for carrying out different treatments thereon, the duration of the residence time which arises out of the spacing in respect of time between two successive transportation steps of the transportation means, depends on the type of treatment which takes up the longest period of time. That may be the drying operation but in many cases it will be another treatment operation, for example the actual printing operation itself.
In other words, the situation is frequently such that it is the printing step that is crucial in regard to fixing the operating cycle of the printing machine and therewith the residence time of an article or a web portion in the drying station, that is to say generally beneath the UV-radiating apparatus. However the resulting residence time does not always have to correspond to the time which is the optimum time for carrying out the drying operation. In that respect it is to be borne in mind that the optimum duration of the drying operation is also dependent on a series of influencing parameters, for example the composition of the ink, but also the thickness of application of the printing ink, as when the ink is applied in a greater thickness, polymerization thereof under the effect of the UV-rays, with a given level of intensity thereof, may take a longer period of time than when the ink is applied in a thinner layer. That means that there is an optimum duration for the drying operation, for each print application. Moreover, in a machine for example for the production of multi-color printing, successive print applications may have different properties in regard to the necessary drying times, for example by virtue of the fact that printing inks are applied in different thicknesses in the individual printing stations of the machine. It will also be appreciated however that, when there is a change in the print image to be produced, which will occur whenever the material to be printed upon changes, the conditions involved for drying the individual printing ink applications may be completely different, and cannot be taken into consideration without particular measures being adopted, as for example it is generally not possible for the UV-radiating device to be switched on and off to set the drying time that represents the optimum duration for the respective application of printing ink to be dried.
Although the use of cold-light mirrors as reflectors for the UV-rays markedly reduces the degree of infra-red radiation which is reflected downwardly towards the article to be treated, the region which is beneath the radiation source, that is to say including the article to be treated, still suffers from a considerable degree of heating due to the residual infra-red radiation which is radiated from the radiation source downwardly on to the parts of the machine at that location, and on to the article to be treated. When dealing with certain articles that phenomenon can give rise to difficulties, for example when dealing with thin-wall bodies such as bottles or other containers in particular of thermoplastic material which suffer from deformation or other unacceptable changes in nature when certain temperature limits thereof are exceeded. For that reason, exceeding a given residence time under the action of the radiation source may be at least undesirable and can actually be harmful and detrimental. Therefore the important consideration is essentially that of limiting the residence time of the article to be treated under the effect of the radiation source to the period which is required for achieving the desired effect, especially as a rise in temperature of the surrounding area, for example the machine frame structure, in the region of the drying station, will also contribute to heating of the articles.
Furthermore however it is a desirable endeavour to limit the period for which the UV-rays act on the article or articles to be treated, to the period of time which is required to dry or polymerize the ink, irrespective of the fact that the articles are subjected to the effect of heat radiation. The wish to avoid UV-irradiation which goes beyond the required degree arises out of the fact that, if they act for an excessively long period of time, UV-rays can also cause undesirable changes in the article, for example in such a way that, under the action of the UV-rays, the color of the applied printing and possibly even the color of the article itself, for example if it is an article of plastic material which contains color pigments, also changes.